Multi-channel cochlear implants have achieved phenomenal success in bringing the world of speech communication to the profoundly deaf. However, even the best implant users experience reduced speech intelligibility in the presence of background noise. This may turn an ordinary social activity such as dining at a restaurant, into a difficult endeavor. Our Hypothesis is that noise influences implant listening in two ways: first, it reduces the dynamic range and changes the loudness growth function and second, it changes the listeners; sensitivity to time-varying aspects of stimuli (such as modulation) Relatively little is known about the psychophysics of implant listening in noisy backgrounds. The overall long-term objective of this research is to achieve in-depth understanding of the effects of such backgrounds on the perception of simple and complex sounds in multi-electrode implant patients. Two fundamental aspects of electrical hearing are addressed in this proposal: 1. Limited dynamic range: Implant listeners have a very limited dynamic range from threshold to uncomfortable loudness level (ULL). We expect that a noise background will raise the threshold and lower the ULL of the tone ("recruitment"). However, the effect of the compressed dynamic range on the shape of the loudness function is unknown. Experiment 1 will measure loudness growth of both tones and AM signals in the presence of wideband, bandpass and lowpass filtered noise. 2.Sensitivity to amplitude modulations: As the degree of spectral detail available to implant listeners is particularly limited compared to normal hearing, sensitivity to temporal fluctuations may play an important role in the benefit derived from the implant. Experiment 2 will investigate the sensitivity of implant listeners to amplitude modulation in the presence of noise with different spectro-temporal characteristics. Experiments will be performed in users of both Nucleus and Clarion systems. For each experimental condition, different degrees of electrode separation between signal and noise will be used in order to address the issue of electrode- or channel- interaction, a third important factor in multi-electrode hearing. It is expected that this research will a) play a role in the development of future generations of auditory implant speech processors and b) provide insight into the fundamental mechanisms of auditory perception in noise.